As a new generation of display, OLED (Organic Light Emitting Diode) displays are widely used due their advantages such as self-emitting, quick response, wide viewing angle, or the like.
An OLED display comprises an array substrate on which a plurality of pixel units is formed in an array. Each of the pixel units comprises two thin film transistors, one serving as the switching thin film transistor functioning as a switch of the pixel unit and the other serving as the driving thin film transistor for driving the array substrate respectively. In the state of art, the active layers of the thin film transistors are usually made of an oxide semiconductor material. Nevertheless, oxide semiconductor materials are sensitive to the light, especially when the external light irradiates the active layer.